Heat transfer devices such as refrigerators, freezers, heat pumps and air conditioning systems are well known. In simple terms such devices typically operate via a cycle wherein a refrigerant of a suitable boiling point evaporates taking heat from its surroundings. After compression, the vapor passes to a condenser where it condenses to a liquid and gives off heat to its new surroundings, before returning to the evaporator via an expansion device and completing the cycle. In addition to the mechanical parts, such as a compressor etc., specially suited materials are needed, including refrigerant, suitable heat transfer materials, sealants to prevent loss of refrigerant, and lubricants to allow for functioning of the movable parts of the device. The lubricant in these devices must have good low temperature flow properties, be thermally stable, provide protection against wear of moving parts such as bearings under load, remove heat from the compressor and seal clearances to ensure efficient compression of gas from low to high pressure.
The refrigerant and the lubricating oil must circulate in the system without undergoing phase separation over a wide temperature range. Typically, the refrigerant and the lubricating oil have low temperature and high temperature regions where they undergo phase separation. Generally, the refrigerant and the lubricating oil undergo is expected to undergo phase separation in the low temperature region at temperatures below 0° C., e.g., −10° C., −20° C. and in some cases lower. Phase separation in high temperature region is expected to be above room temperature, e.g., 50° C., 60° C., or higher. If the phase separation of the refrigerant and the lubricating oil occurs, for example, in the compressor where temperatures are high, movable parts can be insufficiently lubricated, and damage due to baking or the like can lead to shortened lifetime of the device.
On the other hand, excessive miscibility can be problematic. For example, high concentrations of refrigerant in the lubricant can greatly reduce the viscosity of the lubricant, adversely impacting the ability of the lubricant to lubricate and protect parts of the heat transfer device, leading to increased wear, shortened lifetime and lower performance of the device. Dissolved refrigerant in the lubricant can also cause foaming and bubbling of the lubricant mixture as it flows from one area of the compressor to another (e.g., low to high temperature regions). Furthermore, refrigerant dissolved in the lubricant is essentially trapped and taken out of circulation, thereby reducing the capacity of the system.
As efficient functioning of a refrigeration lubricant requires not just proper lubricating properties and appropriate viscosities, but also appropriate compatibility with the refrigerant, changes in refrigerant frequently demand corresponding changes in lubricant.
EP 0 422 182 discloses lubricants prepared by the condensation of pentaerythritol and C6-8 monocarboxylic acids that are almost fully miscible with highly or fully fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane, commonly known as HFC-134a, difluoromethane (HFC-32), trifluoromethane (HFC-23), pentafluoroethane (HFC-125), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1-trifluoroethane (HFC-143a) and the like. Similar lubricants are also disclosed in U.S. Pat. No. 5,964,581.
US 2013/0207023 discloses working fluids for heat transfer devices comprising difluoromethane and a polyol ester lubricant composition having a kinematic viscosity at 40° C. of from about 32 to about 120 cSt comprising linear and branched C5-10 alkylcarboxy esters of a mixture of neopentyl polyols, wherein at least 10 wt % of all neopentyl polyol esters are esters of pentaerythritol oligomers of 4 or more pentaerythritol monomer groups and a majority of the alkylcarboxy groups of the alkylcarboxy esters of neopentyl polyols are pentanoyl groups wherein 15 to 45% of the pentanoyl groups are branched.
U.S. Pat. No. 6,444,626 discloses lubricants comprising poly(pentaerythritol) esters which contain mixtures of pentaerythritol esters, di-pentaerythritol esters, tri-pentaerythritol esters and tetra- and higher oligomeric pentaerythritol esters, which are prepared via a two-step process from mono-pentaerythritol and carboxylic acids, wherein the mono-pentaerythritol is oligomerized during the process. U.S. Pat. No. 8,318,647 discloses refrigeration lubricants comprising select mixtures of carboxy esters of pentaerythritol, di-pentaerythritol and tri-pentaerythritol.
Concern about ozone depletion lead to the replacement of chlorofluorocarbon refrigerants with alternate materials, such as highly or fully fluorinated hydrocarbons, and concern about climate change is leading to replacement, at least in part, of the presently used fluorinated alkanes with fluorinated olefins, particularly hydrofluoro-olefin refrigerants, having a lower global warming potential (GWP). For example, whereas fluorinated alkane refrigerant R-410A has a GWP of 1725, fluorinate alkene, R-1234ze has a GWP of only 6 or less.
Ideally, hydrofluoro-olefin (HFO) refrigerants would serve as “drop-in” replacements for the presently used fluorinated alkanes in refrigeration working fluids. However, it is not uncommon with refrigeration working fluids that changes in the refrigerant, or the conditions under which the working fluid is used, require changes in the lubricant. Even though many hydrofluoro-olefins are structurally similar to commercial saturated hydrofluorocarbons (HFC), simple replacement of saturated hydrofluorocarbons with hydrofluoro-olefins in the existing lubricant compositions has presented a number of challenges. Mixtures of fluorinated olefins with fluorinated alkanes have been developed, as well as miscibility additives for use with fluorinated refrigerants such as hydrofluoro-olefins.
US 2013/0096218 discloses heat transfer compositions comprising mixtures of tetrafluoropropene, difluoromethane and tetrafluoroethane. US 2013/0092869 discloses a composition comprising polyol esters and a mixture of tetrafluoropropene, pentafluoropropene, and trifluoropropyne.
US 2012/0011864 discloses the use of perfluoro-polyethers as additives to improve the characteristics of a wide variety of fluorinated refrigerants including hydrofluoro-olefins.
U.S. Pat. No. 8,603,354 discloses lubricating oil compositions for a refrigerator using as a refrigerant, a fluorine-containing organic compound containing a specific polar structure and having a low global warming potential. Exemplified refrigerants include fluorinated ketones, fluorinated ethers and fluorinated alcohols.
US 2013/0099154 discloses a composition comprising polyol esters and tetrafluoropropene, however only ISO 68 lubricants are exemplified.
The transition to lower global warming potential (GWP) refrigerants is critical to the realization of environmentally sustainable and more energy efficient refrigeration and other heat transfer technologies. Candidates to replace refrigerants R-22 and R-410A in air conditioning and heat pump applications include R-32 (difluoromethane) and hydrofluoro-alkane/hydrofluoro-olefin blends (HFC/HFO blends) with GWPs in the range of 400-650, e.g., blends containing 70% or more R-32 with hydrofluoro-olefin, such as L-41a & b (Honeywell) and DR-5 (DuPont).
Although lubricant is added to the system for lubricating the moving parts of the compressor, it also plays a thermo-fluidic role, impacting capacity and efficiency. For example, lubricants can influence capacity by altering heat transfer coefficients, lowering pressures necessary to reach operating temperatures, and increasing pressure drops. Lubricants also affect efficiency by changing the isoentropic efficiency of the compressor, which would raise or lower the discharge temperature for a given discharge pressure. Lubricants currently used commercially with refrigerants such as R-410A are not necessarily compatible R-32 and HFC/HFO blends under all conditions of use, causing concern over the possibility of inadequate lubrication, poor oil return, and excessive lubricant hold-up in the system.
For a variety of commercial, safety and ecological considerations, it would be desirable to have a working fluid comprising a blend of low GWP refrigerants for applications such as heat pumps and air conditioners for houses, air conditioners for cars and other heat transfer devices, designed to balance cost, flammability and performance. As in other working fluids containing fluorinated refrigerants, the lubricant must exhibit appropriate miscibility with the refrigerant while maintaining a suitable functioning viscosity for the lubricant/refrigerant mixture.
It has been found that working fluids currently used in heat transfer devices can be replaced without loss of operational efficiency with working fluids comprising low GWP refrigerant blends and lubricants having a higher viscosity than presently specified. For example, fluids comprising R-410A and a traditional polyol ester lubricant with an ISO viscosity of 32 can be replaced in heat transfer devices by working fluids of the invention comprising a low GWP refrigerant blend and a lubricant having an ISO grade viscosity of, e.g., 46, 68 or 100 with no loss of operational efficiency. In addition to lower GWP, good lubricant/refrigerant miscibility, excellent lubrication and a high level of wear protection, the proper pairing of lubricant and refrigerant pairing in the inventive working fluid also provides overall heat transfer efficiency improvements.